Heat pump systems use a refrigerant to carry thermal energy between a relatively hotter side of a circulation loop to a relatively cooler side of the circulation loop. Compression of the refrigerant occurs at the hotter side of the loop, where a compressor raises the temperature of the refrigerant. Evaporation of the refrigerant occurs at the cooler side of the loop, where the refrigerant is allowed to expand, thus resulting in a temperature drop. Thermal energy is added to the refrigerant on one side of the loop and extracted from the refrigerant on the other side, due to the temperature differences between the refrigerant and the indoor and outdoor mediums, respectively, to make use of the outdoor mediums as either a thermal energy source or a thermal energy sink. In the case of an air to water heat pump, outdoor air is used as a thermal energy source while water is used as a thermal energy sink.
The process is reversible, so the heat pump can be used for either heating or cooling. Residential heating and cooling units are bidirectional, in that suitable valve and control arrangements selectively direct the refrigerant through indoor and outdoor heat exchangers so that the indoor heat exchanger is on the hot side of the refrigerant circulation loop for heating and on the cool side for cooling. A circulation fan passes indoor air over the indoor heat exchanger and through ducts leading to the indoor space. Return ducts extract air from the indoor space and bring the air back to the indoor heat exchanger. A fan likewise passes ambient air over the outdoor heat exchanger, and releases heat into the open air, or extracts available heat therefrom.
These types of heat pump systems operate only if there is an adequate temperature difference between the refrigerant and the air at the respective heat exchanger to maintain a transfer of thermal energy. For heating, the heat pump system is efficient provided the temperature difference between the air and the refrigerant is such that the available thermal energy is greater than the electrical energy needed to operate the compressor and the respective fans. For cooling, the temperature difference between the air and the refrigerant generally is sufficient, even on hot days.
Under certain operating conditions, frost builds up on a coil of the heat pump. The speed of the frost build-up is strongly dependent on the ambient temperature and the humidity ratio. Coil frosting results in lower coil efficiency while affecting the overall performance (heating capacity and coefficient of performance) of the unit. From time to time, the coil must be defrosted to restore the unit efficiency. In most cases, coil defrosting is achieved through refrigerant cycle inversion. The time at which the coil defrosting occurs impacts the overall efficiency of the unit, since the hot refrigerant in the unit, which provides the desired heat, is actually cooled during coil defrosting.
Conventional units typically use a fixed period between defrosting cycles, irrespective of how much frosting actually occurs within the fixed period. In order to optimize the unit performance while in the heating mode, it is necessary to optimize the time at which coil defrosting occurs.